First, thanks for the informative post and welcome to the forum. It sounds like you might have a 10/100Mbps bottleneck somewhere. What model modem and router do you have? Some routers will have a page that shows what devices are connecting at what speeds. It could be as simple as a crimped ethernet or a bad connector. If that doesn't work, try posting your modem's signal levels. Maybe the outage changed them somehow so they are not sufficient anymore. I will have more information on how to do that once I know what modem you have.
Modem is Motorola MB8611. Router is Netgear Orbi RBR850 with two satellites. I was getting speed test results of 1200Mbps regularly for months until last week when everything got borked.
MB8611
To see the signal levels go to 192.168.100.1 > login > Connections tab. See here for more info. To see the connection stats go to 192.168.0.1/RST_statistics.htm. Should look like picture below.
If it's easier, try a direct connection to the MB8611 and look at your ethernet connection rate from Network connections. Windows Key + R > NCPA.CPL > Right click on Ethernet and left click on status > Look for Speed. If that is Gigabit then run a Cox speed test. However I would post your signal levels first. If you can't upload a image upload it to Imgur and post the link but to upload. To see how to upload, see here.
Connect a computer direct to your modem, (BTW it's a ZOOM modem), and do a speedtest and verify your connected 1000baseTX.
